Breath Figures in Polymer and Polymer Blend Films Spin-Coated inDry and Humid Ambience

Wojciech Madej, Andrzej Budkowski, Joanna Raczkowska,* and Jakub RyszSmoluchowski Institute of Physics, Jagellonian UniVersity, Reymonta 4, 30-059 Krakow, Poland

ReceiVed October 29, 2007. In Final Form: December 18, 2007

We investigate effects of two spin-coating parameters, relative humidity (5% e RH e 80%) in ambient atmosphereand water content (3 wt % e fH2O e 20 wt %) in solution (rich in tetrahydrofuran), on the structure of breath figures(BF) formed in spin-cast films of polar poly(methyl methacrylate) (PMMA) and PMMA mixed with nonpolar polystyrene(PS). Film morphologies, examined with atomic and lateral force microscopy, are analyzed with integral geometryanalysis to yield morphological BF measures. In PMMA, water added to solution has much stronger impact than thatfrom moisture on formed BFs, which could be ordered (with conformational entropy S 0.9-1.0). In PMMA/PS,BFs decorate exclusively polar PMMA domains, resulting in morphologies with two length scales (sub-micrometerBFs and domains >10 m). This suggests a novel strategy for herarchic structure formation in multicomponentpolymer films. In PS/PMMA, BFs are better developed than in pure PMMA spin-coated in identical conditions. Theseobservations show that the air boundary layer facing the spin-cast polymer film (region) is more important than theambient atmosphere.

I. IntroductionBreath figures, although first studied almost 100 years ago,1-3

had not elicited much attention before 1994, when Widawski etal.4 demonstrated how they can be produced in polymer films.This method, which could be called moist-casting, involvespolymer casting from solution under moist ambience. Coolingof solution surface, caused by the solvent evaporation, enablescondensation of water droplets on the surface followed by theirgrowth and spontaneous ordering. Subsequent complete evapo-ration of both solvent and water results in formation of 2D4-17or even 3D4,18-21 arrays of air bubbles (droplet imprints), oftenordered hexagonally, trapped in the polymer matrix.

This technique of film structure formation appears extremelyattractive, as it allows production of porous films (with poresizes easily tunable by preparation conditions from 0.2 to 20

m)4-21 that have numerous potential application in biology,chemistry, and technology.22-31 Unfortunately, it has also somedisadvantages: the majority of experiments carried out for thesolvent slowly evaporating from the drop of solution left on thesubstrate14-17,19,28,30 result in formation of thick polymer layerswith varying thickness, which from a technological point of viewseem to be less convenient than uniform large areas prepared bya fast spin-coating process.20-21 Moreover, most of the previousexperiments were performed for breath figures formed in anatmosphere of high relative humidity (RH > 40%),5,7,15-16,19,28,32which should be avoided in high-tech laboratories. Only veryrecently, Park and Kim20 have shown that breath figures can beobtained also in a drier atmosphere (RH ) 30%) by spin-castingpolymer films from solutions containing small amounts of water.

In this paper, we present systematic studies on the influenceof two spin-coating parameters, relative humidity in ambientatmosphere and water content fH2O in the solution, both variedin a wide range (5% e RH e 80% and 3 wt % e fH2O e 20 wt%), on the breath figure formation in the spin-cast polymer films.The films are composed of poly(methyl methacrylate) (PMMA).Film morphologies are analyzed numerically with the integralgeometry approach33-36 to yield morphological measures such

* Corresponding author: E-mail: raczkowska@if.uj.edu.pl.(1) Rayleigh, L. Nature 1911, 86, 416.(2) Aitken, J. Nature 1911, 86, 516.(3) Baker, J. T. Philos. Mag. 1922, 56, 752.(4) Widawski, G.; Rawiso, M.; Francois, B. Nature 1994, 369, 387.(5) Karthaus, O.; Maruyama, N.; Cieren, X.; Shimomura, M.; Hasegawa, H.;

Hashimoto, T. Langmuir 2000, 16, 6071.(6) Peng, J.; Han, Y.; Fu, J.; Yang, Y.; Li, B. Macromol. Chem. Phys. 2003,

204, 125.(7) Peng, J.; Han, Y.; Yang, Y.; Li, B. Polymer 2004, 45, 447.(8) Hayakawa, T.; Horiuchi, S. Angew. Chem., Int. Ed. 2003, 42, 2285.(9) Francois, B.; Pitois, O.; Francois, J. AdV. Mater. 1995, 7, 1041.(10) Francois, B.; Ederle, Y.; Mathis, C. Synth. Met. 1999, 103, 2362.(11) Stenzel, M. H. Aust. J. Chem. 2002, 55, 239.(12) Pitois, O.; Francois, B. Eur. Phys. J. B 1999, 8, 225.(13) Nishikawa, T.; Ookura, R.; Nishida, J.; Sawadaishi, T.; Shimomura, M.

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21, 3235.(30) Bolognesi, A.; Mercogliano, C.; Yunus, S.; Civardi, M.; Comoretto, D.;

Turturro, A. Langmuir 2005, 21, 3480.(31) Pintani, M.; Huang, J.; Ramon, M. C.; Bradley, D. D. C. J. Phys.: Condens.

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3517Langmuir 2008, 24, 3517-3524

10.1021/la703363a CCC: $40.75 2008 American Chemical SocietyPublished on Web 02/23/2008

as average pore size (2R) and surface area fraction () coveredby the pores: It is shown that water added to the solution hasmuch stronger impact on the resulting breath figures than thewater originating from ambient moisture. In contrast, no porousstructures were observed in spin-cast films of polystyrene (PS).

In addition, we present and analyze numerically breath figuresformed in the spin-coated polymer blend films. The films arecomposed of (polar) PMMA and (nonpolar) PS. In contrast tosimilar experiments performed previously for drop cast37-41 andspin-cast42 blend films, the dropletlike patterns are formed hereselectively only in the domains rich in one (polar) polymer:Sub-micrometer pores decorate large (>10 m) PMMA-richsurface regions, which stand out of plain PS-rich surface areas,resulting in hierarchic film morphology.

Multicomponent polymer films with hierarchic structures canbe potentially useful for different technologies. For instance,they can be applied to fabricate waveguides based on photoniccrystals,43 macroelectronic displays composed of structuredorganic light emitting diodes,44,45 or plastic circuitries46,47integrated with protein-based biosensors.48 This motivated ourstudies, which resulted in a new strategy for hierarchic self-organization in multicomponent polymer films, where blends oftwo homopolymers (with different polarities) are used ratherthan homopolymer multilayers49 or block copolymers.43

II. Experimental SectionSample Preparation. Polymers used in this work were poly-

(methyl methacrylate) (PMMA; molecular weight Mw ) 14 400,polydispersity index Mw/Mn ) 1.04) and polystyrene (PS; Mw )18 000, Mw/Mn ) 1.04), purchased from Polymer Standard Service(Mainz, Germany) and used as obtained. The films used to studyformation of breath figures were spin-coated from polymer solutionsof PMMA, PS, or their binary mixture (with 15:85 PMMA:PS massfraction) dissolved in tetrahydrofuran (THF) admixed with variousamounts of water (total polymer concentration cp ) 100 mg/mL).

The samples were spin-cast for 20 s (spin-casting speed ) 1000rpm) onto silicon wafers covered with a native oxide (SiOx) using

(34) Raczkowska, J.; Bernasik, A.; Budkowski, A.; Rysz, J.; Kowalski, K.;Lekka, M.; Czuba, P.; Lekki, J. Thin Solid Films 2005, 476, 358.

(35) Mecke, K. R. Int. J. Mod. Phys. B 1998, 9, 861.(36) Michielsen, K.; De Raedt, H. Phys. Rep. 2001, 347, 461.(37) Cui, L.; Wang, H.; Ding, Y.; Han, Y. Polymer 2004, 45, 2424.(38) Cui, L.; Peng, J.; Ding, Y.; Li, X.; Han, Y. Polymer 2005, 46, 5334.(39) Cui, L.; Han, Y. Langmuir 2005, 21, 11085.(40) Cui, L.; Xuan, Y.; Li, X.; Ding, Y.; Li, B.; Han, Y. Langmuir 2005, 21,

11696.(41) Mitov, Z.; Kumacheva, E. Phys. ReV. Lett. 1998, 81, 3427.(42) Gliemann, H.; Almeida, A. T.; Petri, D. F. S.; Schimmel, T. Surf. Interface

Anal. 2006, 39, 1.(43) Park, C.; Yoon. J.; Thomas, E. L. Polymer 2003, 44, 6725.(44) Fichet, G.; Corcoran, N.; Ho, P. K. H.; Arias, A. C.; MacKenzie, J. D.;

Huck, W. T. S.; Friend, R. H. AdV. Mater. 2004, 16, 1908.(45) Pintani, M.; Huang, J.; Ramon, M. C.; Bradley, D. D. C. J. Phys.: Condens.

Matter 2007, 19, 016203.(46) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.;

Wu, W.; Woo, E. P. Science 2000, 290, 2123.(47) Haberko, J.; Raczkowska, J.; Bernasik, A.; Rysz, J.; Budkowski, A.;

uz ny, W. Synth. Met. 2007, 157, 935.(48) Zhang, Y.; Wang, C. AdV. Mater. 2007, 19, 913.

(49) Morariu, M. D.; Voicu, N. E.; Schaffer, E.; Lin, Z.; Russell, T. P.; Steiner,U. Nat. Mater. 2002, 2, 48.

Figure 1. Surface topography (AFM; A, C) and composition (LFM;B, D) recorded for PMMA film spin-coated from the solution withhigh water content (fH2O ) 9 wt %) in dry (RH ) 25%; A, B) andhumid (RH ) 80%; C, D) atmosphere. The gray level depicts theheight (AFM) or LFM signal scale.

Figure 2. AFM image (A) and corresponding LFM micrograph (C)of the sample region with spin-cast PMMA film (fH2O ) 9 wt %,RH ) 25%) and with bare substrate. The line in part A indicatesthe location where the cross-section (B) was taken.

3518 Langmuir, Vol. 24, No. 7, 2008 Madej et al.

a coater (KW-4A, Chemat Technology) extended with a homemadesystem50 to monitor and adjust relative humidity (RH) in the coatingbowl. RH was equal to 5, 25, 50, or 80% for films composed ofPMMA and PS/PMMA blends and 5, 25, or 50% for PS layers.Water weight fraction (fH2O) in THF was equal to 3, 6, 9, or 20 wt% for films prepared from PMMA solution; fH2O ) 3, 6, or 9 wt %for PS/PMMA blend films; and fH2O ) 9 wt % for PS layer. Additionalsamples composed of PMMA (RH ) 5, 25, 50%; fH2O ) 3, 6, 9 wt%) were spin-coated with 1 ) 3000 rpm.

Morphology Examination. Surface topography and lateraldomain arrangement in the studied polymer films were examinedby atomic (AFM) and lateral (LFM) force microscopy (The AcademiaSystem, Nanonics Imaging Ltd., Israel) working in contact mode.50Average film thickness was determined after partial film removalby a scalpel scratch, whereas polymer phases and their verticalarrangement were identified after selective polymer dissolution(resulting from film immersion for 60 s in acetic acid, for PMMA,or in cyclohexane, for PS).

Image Analysis. The topographic (AFM) images were examinednumerically with the integral geometry approach33-36 using thesoftware developed in our laboratory.33,34 For each AFM image[with bimodal height (h) distribution centered at h1 and h2], the

surface coverage (F) by the elevated regions [i.e., with local heighth > (h1 + h2)/2] and their Euler characteristic (E) (connectivity)were calculated33,34 using the algorithm of ref 36 (simpler butequivalent to that of ref 35). E is defined as the difference betweenthe number of separated elevated and depressed regions normalizedby the analyzed area. The Minkowski measures F and E of the filmswith breath figures were used to define and evaluate (see the section3.1) the morphological measures of individual pores, such as theiraverage size (2R) and (fractional) surface coverage ().

Ordering of individual pores was quantitatively analyzed byVoronoi polygon construction.20,32

III. Results and Discussion3.1. Breath Figure Formation in Polymer Films. Repre-

sentative images of surface topography (AFM) and composition(LFM) of investigated PMMA films are presented in Figure 1,for a sample prepared by spin-casting in moist atmosphere (RH) 80%) from the solution with high water content (fH2O ) 9 wt%).

Close-packed circular holes, which can be observed on thesurface (Figure 1A,C), correspond to the imprints of water dropletscondensed on the surface in the course of spin-casting.18,20,21,25,38The vertical extent of these pores is much smaller than film

(50) Jaczewska, J.; Budkowski, A.; Bernasik, A.; Raptis, I.; Raczkowska, J.;Goustouridis, D.; Rysz, J.; Sanopoulou, M. J. Appl. Polym. Sci. 2007, 105, 67.

Figure 3. LFM images reflecting porous surface structures formed in PMMA films spin-cast under different conditions, with varied relativehumidity in ambient atmosphere RH ) 5% (A, E, I, M), 25% (B, F, J, N), 50% (C, G, K), and 80% (D, H, L) and the water content in thesolution fH2O ) 3 wt % (A-D), 6 wt % (E-H), 9 wt % (I-L), and 20 wt % (M, N).

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thickness (see the next paragraph). Therefore, the frictionalcontrast, clearly visible in the LFM images (Figure 1B,D), is notdue to the substrate exposed by the pores much smaller thanpolymer films. Instead, it can be attributed to the modificationof polymer material in the regions covered by condensed waterdroplets, possibly reflecting interfacial polymer layers (polymerbags) formed around the droplets and stabilizing them.12,25 Analternative explanation involving the interfacial activity ofamphiphilic PMMA, with polar (COOCH3) and nonpolar groups(CH2, R-CH3), is less plausible for the atactic PMMA polymerused.51 Figure 1C,D shows also that the frictional contrast (LFM)is less sensitive than topography (AFM) to surface undulations.For this reason, LFM micrographs would be used to illustratethe lateral features of the studied porous film structures.

Vertical Film Structure. Figure 2 shows a typical samplewith a spin-cast PMMA film, partially removed by a scalpelscratch: We notice at once that the pores are formed in theupper, surface-adjacent part of the polymer layer (see the cross-section in Figure 2B). The density of the solvent (THF with F) 0.889 g/cm3) is smaller than that of water, which should promotemultilayers of air bubbles.4,18-21 However, the miscibility ofTHF and water prevents such a scenario, as the condensed waterdroplets, which sink down into the film, coalesce and mix withthe THF-rich solution.20 This mechanism is ineffective near thesurface, where polymer concentration is higher and solidification

is faster. The vertical (400 nm) extent of pores (Figure 2) iscomparable with their lateral radius (R ) 355 ( 52 nm, determinedas described later) indicating that the bubbles, remaining fromwater droplets, open up due to surface tension effects.25 Poredimensions are much lower than film thickness determined fromcross-sections taken on the polymer film edge (1450 ( 50 nmfor the sample of Figure 2).

The thickness of spin-cast PMMA films depends strongly onsolution composition: For instance, for the films coated at dryambience (RH ) 25%), thickness grows almost linearly withwater content in the solution: d ) 550 ( 50, 900 ( 80, and 1450( 50 nm for fH2O ) 3, 6, and 9 wt %, respectively. This can berelated with an intermediate stage of spin-casting, when the flowof solution, caused by a balance between centrifugal and viscousforces, decreases film thickness and controls its final valueadjusted by spinning speed () and solution composition.33,50,52As water is more viscous ( ) 1.005 cP) than THF ( ) 0.486cP), an increased water fraction in THF-rich polymer solutioninduces larger viscous forces and hence thicker final films.

Lateral Film Structure. LFM images reflecting the effectsof two spin-coating parameters, relative humidity (RH) in theambient atmosphere and water content (fH2O) in the solution, onthe breath figures formed in spin-cast PMMA films are presentedin Figure 3. For the layers cast from solutions with low watercontent (fH2O ) 3 wt %; Figures 3A-D), porous structures areproduced only in humid atmosphere (RH ) 50, 80%; Figure3C,D). An increase of the water fraction in the solution promotesthe formation of breath figures in dry ambience (RH ) 5, 25%;Figure 3E,F,I,J,M,N). The pores observed for fH2O ) 6 wt % andRH ) 5% (Figure 3E) do not cover the surface uniformly;however, the patterns formed at higher humidity (Figure 3F-H)are homogeneously distributed and better ordered. Furtherincrease of water content (fH2O ) 9 vs 6 wt %; Figure 3, partsI-L vs E-H) leads to continuous improvement in the uniformityof breath figures. For solutions with very high water content(fH2O ) 20 wt %), deposition of polymer layers in humidatmosphere (with RH > 25%) is prevented by dewetting ofpolymer-rich solution from the substrate.20

Integral Geometry Analysis. Breath figure formation underdifferent spin-coating conditions was analyzed numerically withthe integral geometry approach.33-36 For each AFM image,directly corresponding to one of the morphologies in Figure 3,two Minkowski measures were calculated, reflecting the surface(fractional) coverage, F, by the elevated regions and theirconnectivity (the Euler characteristic), E. These Minkowskifunctionals are used to determine the morphological measuresof individual pores, such as their average radius R and (fractional)surface coverage :

The results of such analysis, presented in Figure 4, characterizesub-micrometer pores with the average size 2R varying from

(51) Tretinnikov, O. N.; Ohta, K. Langmuir 1998, 14, 915.(52) Raczkowska, J.; Cyganik, P.; Budkowski, A.; Bernasik, A.; Rysz, J.;

Raptus, I.; Czuba, P.; Kowalski, K. Macromolecules 2005, 38, 8486 and referencestherein.

Figure 4. Surface area fraction covered by the pores (A) andtheir average radius R (B), calculated with the integral geometryapproach for spin-cast PMMA films. Evaluated error bars (not shown)are

130 ( 19 to 850 ( 100 nm, covering up to 35% of the totalsurface area. Such coverage indicates that polymer ribbons aroundthe pores form an important element of the observed breath figures.

Pore radius R increases significantly (Figure 4B) with waterfraction fH2O in the solution, rising up to 9 wt %, independentlyof ambient humidity 5% e RH e 80%. The same is true (Figure4A) for the area fraction covered by pores, but for humidityin the range 5% e RH e 50%. Similar growth of pore size,promoted by water admixed to the solution, was observed byPark and Kim20 for films of cellulose acetate butyrate spin-castat RH ) 30%.

In turn, the effects of relative humidity on both morphologicalpore measures (for constant fH2O) are much weaker (Figure 4).We conclude that water added to the solution has much strongerimpact on the resulting breath figures than the water originatingfrom ambient moisture. This can be related with the de Gennesmodel53 of evaporating film, which makes a clear distinction

between the air boundary layer, facing the film, and ambientatmosphere. Droplet condensation is affected directly by thehumidity in the air boundary layer, modified more strongly bythe water from adjacent solution than from distant atmosphere.Another relevant mechanism relates larger pores (i.e., increasedR and ) with longer growth of water droplets, which is possiblefor thicker spin-cast films, which dry slower. As discussed above,the thickness of spin-cast PMMA films indeed increases withwater content in the solution.

A very similar mechanism can explain why the pores observedfor PMMA films coated with higher spinning speed of 1 )3000 rpm (see Figure S1, Supporting Information) are smallerthan those obtained for ) 1000 rpm and presented in Figure3. Centrifugal forces, important for the intermediate stage ofspin-coating, increase with and result in thinner final polymerfilms, which dry faster. The shortened time available for droplet

(53) de Gennes, P. G. Eur. Phys. J. E. 2001, 6, 421.

Figure 5. AFM images (A, C, E, G) and corresponding Voronoi polygon constructions (B, D, F, H) obtained for the pores (with variouscoordination number) formed on PMMA films spin-cast under different conditions: fH2O ) 6 wt % (A, B) and 9 wt % (C-H), RH ) 5%(E, F), 25% (C, D), and 50% (A, B, G, H). Pores with six, seven, four, and five nearest neighbors are colored by four sequential gray levels(from almost white to black).

Figure 6. Surface topography (AFM; A, C) and composition (LFM;B, D) recorded for PMMA/PS film spin-coated in dry atmosphere(RH ) 25%) from the solution with water content fH2O ) 3% (A,B) and 6 wt % (C, D).

Figure 7. AFM (A, C) and LFM (B) images of spin-cast PMMA/PS film (fH2O ) 6 wt %, RH ) 50%), prior to (A, B) and after (C)selective dissolution of PMMA-rich phase. The lines in parts A andC mark the cross-sections, which are compared in part D.

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growth, leading to smaller pores, cannot be compensated bymore effective surface cooling, which should increase dropletcondensation.

For the samples cast from solutions with the highest waterfraction (fH2O ) 20 wt %), the pore measures and R do notfollow the growth with fH2O discussed above for lower wateradmixtures (Figure 4). This can be attributed to the film dewettingprocess modifying the conditions for breath figure formationand finally destroying polymer films for RH > 25%.

Conformational Entropy Analysis. Ordering of individualpores is analyzed quantitatively by the construction of Voronoipolygons20,32 presented in Figure 5 for the films with the highestsurface coverage by the pores. Figure 5A,C,E,G shows AFMimages, whereas Figure 5B,D,F,H depicts the correspondingVoronoi polygons, which surround the center of each pore andbisect the lines pointed toward its (four, five, six, or seven) nearestneighbors. The pores with six, seven, four, and five nearestneighbors are colored in Figures 5B,D,F,H by four sequentialgray levels (from almost white to black). The correspondingprobabilities Pi of the pores being formed with i nearest neighborsare listed in Table 1. They yield also the conformational entropy(S) of breath figures: S ) -Pn ln Pn.20,32

Analysis of the values in Table 1 clearly shows the highestprobability of the pores having six nearest neighbors (P6 ismaximal for fH2O ) wt 9%, RH ) 25%), reflecting preferentialhexagonal ordering. Moreover, conformational entropy valuesS ) 0.93-1.03 (Table 1) obtained for the analyzed images aresignificantly smaller than the value S ) 1.71 obtained for randomlydistributed pores,32 suggesting formation of breath figures withfairly well ordered pores. Ordering, concluded here for PMMA(Figure 5), is similar to that obtained by Park and Kim20 forspin-coated cellulose acetate butyrate (S ) 0.86). Lower valuesof conformational entropy (S 0.4-0.5) can be obtained onlyfor slowly evaporating solution droplets.20

3.2. Breath Figure Formation in Polymer Blend Films.Representative images of surface topography (AFM) andcomposition (LFM) of films spin-coated from PMMA/PS blendare shown in Figure 6. Surprisingly, two types of surface domainscan be distinguished in the micrographs: elevated (see Figure6A,C) regions with breath figures clearly visible and lower, almostplain areas (with sporadic holes). AFM and LFM micrographsof the porous structure formed in the higher surface areas recallthose discussed earlier for pure PMMA films (e.g., Figure 1).In turn, microscopic analysis of spin-coated pure PS films (seeFigure S2, Supporting Information) reveals no signs of breathfigures, similarly to the depressed domains of PMMA/PS films.These morphological similarities suggest that higher and lowerregions of PMMA/PS films are rich in PMMA and PS,respectively. Observed hierarchical morphology, with large (>10m) domains and sub-micrometer pores, is a result of twoprocesses: blend demixing and selective formation of breathfigures on the areas rich in more polar PMMA.50 Logically,phase separation must precede pore structure creation. Only onsingular micrographs were elliptical pores observed (Figure S3,Supporting Information) due to the phase coarsening notterminating before the completion of breath figures.51 Figure6A,B shows again that the frictional contrast (LFM) is lesssensitive than topography (AFM) to surface undulations. Forthis reason, LFM micrographs are used again to illustrate thelateral features of porous film structures.

Vertical Film Structure. Comparable surface coverage byboth phases of PMMA/PS films (Figure 6) seems to contradictthe (nominal) blend composition (15:85 w:w PMMA:PS; see theExperimental Section). This can be explained by a vertical domainarrangement with the PS-rich phase dominant near the substrate.To verify both the identity of surface domains and suggestedvertical film structure, microscopic observations were combined

Figure 8. LFM images reflecting porous surface structures formed in PS/PMMA blend spin-cast under different conditions with variedambient humidity RH ) 5% (A, E, I), 25% (B, F, J), 50% (C, G, K), and 80% (D, H, L) and the water content in the solution fH2O ) 3 wt% (A-D), 6 wt % (E-H), and 9 wt % (I-L).

3522 Langmuir, Vol. 24, No. 7, 2008 Madej et al.

with selective dissolutions of the phase rich in PMMA (Figure7) and PS (Figure S4, Supporting Information).

Selective dissolution experiments (cf. Figure 7A,C) clearlyconfirm that the droplet-like patterns are formed only in thedomains rich in one polymer (PMMA). This is a new situation,contrasting with previous reports on blend demixing combinedwith evaporation effects in drop-cast37-41 and spin-cast42 films.It differs also from the most recent of these studies focused onthe superposition of phase separation and breath figure forma-tion.38,40,42 The main reason for this new behavior is nonpolarpolymer (PS) mixed with polar molecules (PMMA) and veryfast film drying. Spin-coating is too fast to allow for breathfigure formation in PS (Figure S2), observed only in slow drop-casting processes7,14,15,18 (also for PS without polar end groups7).In addition, the characteristic scales of pore patterns and thecoarsening phases are dissimilar for spin-casting but could becomparable for drop casting, leading to a one-scale filmstructure38,40 instead of a hierarchic morphology as in Figure 6.

Microscopy combined with dissolution (Figure 7) verifies alsothe vertical film structure, with PMMA-rich domains located onthe top of the PS-rich layer wetting the SiOx substrate, consistentwith the blend composition (15:85 w:w PMMA:PS). Filmmorphology of solution cast mixtures, often examined for thePMMA/PS system,41,54-60 is a result of a few competingmechanisms and depends on several factors, such as substrate,blend composition, solvent, film thickness, and polymer molecularweight.41,54-60 Blend film structure similar to that in Figure 7

was observed previously by Mitov and Kumacheva41,60 for themixture with similar composition (1:9 w:w PMMA:PS) spin-cast (from toluene) onto quartz glass.

Multiple mutually coupled mechanisms effective during spin-coating of PMMA/PS blends are not completely resolved, butthe main stages of film structure formation have beenrecognized:41,54-60 First, phase separation, initiated by evaporatingsolvent, takes place simultaneously with a radial liquid flow,which controls final film thickness. Second, transient multilayerphase arrangement is formed and then broken up by interfacialinstability into a coarsening lateral surface phase structure (Figure7). Third, both the phase coarsening and film thinning areterminated when polymer molecules are no longer mobile. Breathfigure formation, selective for PMMA surface domains, must beinitiated during the second period (Figure S3) and terminatedduring the third stage, when PMMA (less soluble in THF) vitrifiesearlier than PS to form elevated domains (Figure 6).

Lateral Film Structure. LFM images illustrating the effectsof spin-coating parameters, such as relative humidity and waterfraction in the solution, on the pore structures formed in PMMA/PS blend films are presented in Figure 8. Before we start todiscuss the recorded breath figures, we notice that Figure 8A(the lowest values of both fH2O and RH) shows only very small(darker) domains rich in PMMA, while the other images of Figure8 indicate a much higher surface coverage by the PMMA phase.This can be related with the effect of water uptake by the morehygroscopic polymer (PMMA),50 increasing its polarity andinteraction tension with the other phase and modifying the finalblend film morphology.56 Interfacial instabilities would be moreeffective for higher fH2O and RH, resulting in more lateral phasearrangement (cf. Figure 8, parts A and B-L).

For the layers cast from solutions with the low water content(fH2O ) 3 wt %; Figures 8A-D), uniform regions with porousstructures are formed on PMMA-rich domains, except forextremely dry conditions (RH ) 5%, Figure 8A). This is incontrast to the situation for pure PMMA films (cf. Figure 3A-D),where uniformly distributed pores appear only at RH ) 80%.In dry ambience of blend films, an increase of water fraction (tofH2O ) 6 wt %) promotes breath figures (RH ) 5%; Figure 8E)and makes them more pronounced (RH ) 25%; Figure 8F). Forhigher humidity, the pores are similar to those obtained for lowerfH2O (cf. Figure 8, parts G,H and C,D). Finally, for the solutionswith even higher water content (fH2O ) 9 wt %), irregular structuresobserved in the regions without the pores (see, e.g., Figure 8J)suggest dewetting of polymer-rich solution from the substrate,61noted earlier for pure PMMA films but at higher fH2O ) 20 wt%. Except for the fH2O ) 9 wt % series, the pore structuresrecorded for (PMMA-rich domains of) PMMA/PS films are moredeveloped than those obtained for pure PMMA layers, when

(54) Raczkowska, J.; Budkowski, A.; Rysz, J.; Czuba, P.; Lekka, M.; Bernasik,A. J. Nanostruct. Polym. Nanocomposit. 2005, 1, 23 and references therein.

(55) Walheim, S.; Boltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macro-molecules 1997, 30, 4995.

(56) Sprenger, M.; Walheim, S.; Budkowski, A.; Steiner, U. Interface Sci.2003, 11, 225.

(57) Ton-That, C.; Shard, A. G.; Bradley, R. H. Polymer 2002, 43, 4973.(58) Heriot, S. Y.; Jones, R. A. L. Nat. Mater. 2005, 4, 782.(59) Dalnoki-Veress, K.; Forrest, J. A.; Stevens, J. R.; Dutcher, J. R. Physica

A 1997, 239, 87.(60) Kumacheva, E.; Li, L.; Winnink, M. A.; Shinozaki, D. M.; Cheng, P. C.

Langmuir 1997, 13, 2483.(61) Film detwetting for the fH2O ) 9 wt % series is also suggested by the

enhanced LFM contrast between homogenous areas and the PMMA regionspatterned with pores (cf. Figure 8, parts I-K and E-G). However, it can beargued that the friction coefficient between the tip and polar polymer can beaffected by swelling due to water uptake56 (higher for the fH2O ) 9 wt % series).

Figure 9. Area fraction of PMMA surface domains, covered bythe pores (A) and the average pore radius R (B), calculated with theintegral geometry approach for spin-cast PMMA/PS blend films.Evaluated error bars (not shown) are

spin-coated at identical conditions (cf. Figures 8B-H and 3B-H,respectively).

Integral Geometry Analysis. Breath figures formed inPMMA/PS blend films were analyzed numerically with theintegral geometry approach in terms of the average pore radiusR and the area fraction of PMMA surface domains coveredby the pores. The results of such an analysis are presented inFigure 9. We notice, that R and increase with the water fractionfH2O in the solution rising up to 6 wt %, but such rise, strong at(constant) RH ) 5%, disappears at more humid ambience. Thisis in contrast to the strong growth of both measures with fH2O,observed for pure PMMA films. In addition, systematic humidityeffects on R and values are visible only for PMMA/PS layersspin-coated from the solution with fH2O ) 3 wt % and reappearfor fH2O ) 9 wt % when layer dewetting becomes important.

Very interesting is also the comparison of the pore measurescalculated for the PMMA (Figure 4) and PMMA/PS films (Figure9) prepared at identical spin-coating conditions (5 e RH e 80%,3 e fH2O e 6 wt %). Both the pore size 2R and the fractionalcoverage are much larger for the PMMA domains of PS/PMMA blends than for pure PMMA films. This can be explainedby the ability of polar PMMA to absorb water at least two timesmore strongly than for nonpolar PS.50 Water, present originallyin the blend solution, accumulates in the minor PMMA-richdomains during demixing. Therefore, later on the water evapora-tion from PMMA surface regions increases locally above theaverage value. This leads to higher humidity in the nearby airboundary layer (see previous Section) and finally to the porestructures more developed for PMMA-rich blend domains thanfor pure PMMA films.

ConclusionsThin polymer films are often deposited by the technologically

attractive technique of spin-casting. In this paper, we analyzebreath figures formed in polymer (PMMA) and polymer blend(PS/PMMA) films spin-coated from a solvent (tetrahydrofuran)admixed with water. First, systematic studies on the effect oftwo parameters, RH in ambient atmosphere and fH2O in thesolution, on the final film structures are performed. Final filmmorphologies with breath figures are analyzed numerically, withthe integral geometry approach, in terms of average pore sizeand area fraction of the surface covered by the pores.

Breath figures, observed for the films prepared from PMMAsolutions, depend strongly on both analyzed parameters: RHand fH2O. Structures with clearly visible pores are formed evenin dry atmosphere for the solutions with fH2O g 6 wt %. Thepores are fairly well ordered, as confirmed by conformationalentropy analysis. This effect is strengthened in the atmospherewith high RH. Sub-micrometer pore size 2R and surface coverage increase considerably with water fraction fH2O, while humidityeffects are much weaker. For very high water content (fH2O )20 wt %), a dewetting process prevents formation of polymerlayers where breath figures might appear. No breath figures areobserved for PS films spin-coated in conditions comparable tothose used for PMMA.

For the films spin cast from PS/PMMA blend solutions, breathfigures are visible only in PMMA domains prominent on thebackground of nearly plain PS-rich phase. In contrast to the purePMMA, here the pores are formed in dry atmosphere even forthe solutions with low water content (fH2O ) 3 wt %). Moreover,both parameters, pore size 2R and pore coverage of PMMAdomains, are larger than for pure PMMA spin-cast at identicalconditions (fH2O e 6 wt %). This suggests preferential wateraccumulation in the PMMA phase of the blend, leading to thelocal increase of water content during coating. Selective formationof breath figures on PMMA areas, subsequent to blend demixing,leads to hierarchic film structures with sub-micrometer poresand large (>10 m) surface domains.

The observations made for spin-coated polymer (PMMA) andpolymer blend (PMMA/PS) films show that ambient atmosphereis less important for pore structure formation than the air boundarylayer facing the evaporating (PMMA) film or even local (PMMA-rich) film regions. This confirms the de Gennes model ofevaporating film.53 As the local control of film regions can beexecuted during spin-coating,33,52 it should be in principle possibleto coat polymer surfaces with templated areas covered by breathfigures.

From the viewpoint of potential applications, this work suggestsa new approach to fabricate multicomponent polymer films withhierarchic morphology.43,48 Spin-coating the blends of polar andnonpolar polymers results in surface domain structure and breathfigures decorating selectively the domains rich in the more polarpolymer. The morphology of both the domains and the breathfigures can be controlled with prepatterned substrates47,52,62 andcoating conditions, respectively. Therefore, such a self-organiza-tion strategy could be potentially useful for fabricating novelhigh-tech devices, such as photonic waveguides,43 OLEDdisplays,44,45 or protein chips.48

Acknowledgment. This work was partially supported by theReserve of the Faculty of Physics, Astronomy, and AppliedComputer Science of the Jagellonian University. One of us (J.Raczkowska) is grateful to the Foundation for Polish Science forfinancial support.

Supporting Information Available: Additional AFM imagespresenting formation of breath figures in layers composed of PMMAspin-cast with 1 ) 3000 rpm (Figure S1); the results of experimentsperformed for pure PS solution, cast from solution with fH2O ) 9 wt %in the atmosphere with relative humidity RH up to 50% (Figure S2);micrographs presenting elliptical pores (Figure S3) as well as the resultsof selective dissolution of PS-rich phase in PS/PMMA films (FigureS4). This material is available free of charge via the Internet athttp://pubs.acs.org.

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(62) Boltau, M.; Walheim, S.; Mlynek, J.; Krausch, G.; Steiner, U. Nature1998, 391, 877.

3524 Langmuir, Vol. 24, No. 7, 2008 Madej et al.